Hot/cold packs have been employed in one form or another for centuries. One of the earliest modern forms of cold pack still in use today is no more than water ice cubes inside a plastic bag. The other example of a well known pack is the hot water bag. These common devices have been known and utilized for over a century.
The traditional hot water/ice bag has to a substantial degree been replaced by the use of high viscosity water based gels instead of ice water, the gels have a suppressed freeze point to maintain flexibility when frozen. Such devices have disadvantages.
Conventional hot/cold gel packs are very heavy and shipping costs are very expensive. Freight charges can exceed the product costs, the further the travel distance the greater the transport costs.
Another disadvantage of gel based approaches is that in order to achieve flexibility manufacturers must add various additives to lower the freezing point of the gel so that it can flexible when frozen. These additives can be common sodium chloride or propylene glycol added to the mostly water based gel. A great deal of effort is spent blending the proper ratios of gel to water to additives to achieve the desired gel freeze suppressant.
A third disadvantage is that modern hot/cold gel packs cannot be placed directly in the microwave to heat the packs.
This can be very dangerous. Modern gel hot/cold packs are contained typically in non-porous wrappers that cannot breathe. If a gel pack is excessively microwaved it can explode. This occurs when the water component is turned to vapor when internal gel temperatures exceed 212 degrees Fahrenheit. The pack will swell as the water turns to steam. Pressure will increase until the wrapper bursts. This is extremely dangerous if the superheated gel comes in contact with the skin.
A fourth disadvantage of modern hot/cold packs is that when they are wholly frozen they can cause frostbite when in direct contact with the skin or can damage products that cannot be subjected to sub zero freezing temperatures. It is believed that virtually every hot/cold gel pack sold today states that one must place a barrier between the pack and skin to prevent frostbite since the surface temperature is well below 32 degrees Fahrenheit.
A fifth disadvantage of all known modern hot/cold packs is that overall thermal performance is decreased due to the addition of freeze suppressing agents. These chemicals lower the freeze point to achieve a flexible gel at temperatures below freezing, however this also decreases the duration of cooling by the pack since the gel never freezes (becomes solid).
A sixth disadvantage of modern hot/cold gel packs is that the entire pack becomes useless if it is punctured or torn. The entire contents of the pack can escape the outer container.
A seventh disadvantage of modern hot/cold gels is that most altered gel structures employed are of a very slippery, slimy or corrosive nature and cannot easily be disposed of. In addition, they can contain harmful substances. These gels are not water soluble.